Pile exmplsolu

PILE FOUNDATION.
One or more of the followings:
(a)Transfer load to stratum of adequate capacity
(b)Resist lateral loads.
(c)1
Transfer loads through a scour zone to bearing stratum
(d)Anchor structures subjected to hydrostatic uplift or overturning
s
Pile
pQ
Q
uQ
spu QQQ +=
1
Check setlements of pile groups
R. KHERA 1 PileExmplSolu.doc04/14/03 PileExmplSolu.doc

Do not use piles if:
Driving may cause damage to adjacent structures, soil may heave
excessively, or in boulder fields.
Pile Installation
Hammers
Drop hammer1
Pile
Pile cap
Cushion
Ram
Drop hammer
Cushion
1
Very noisy, simple to operate and maintain , 5-10 blows / minute, slow driving, very large drop, not suited for end bearing piles,
used on Franki piles

Exhaust
Intake
Ram
Pile cap
Cushion
Pile
Single acting hammer
Cushion
Double acting1
:
Differential acting2
Diesel3
Vibratory4
Jacking
Predrilling or Jetting
1
Uses pressure for up stroke and down stroke. Design limits prevent it to deliver as much energy as single acting, but greater speed,
used mostly for sheet piles
2
Has two pistons with different diameters, allowing it to have heavy ram as for single acting and greater speed as double acting
3
Difficult to drive in soft ground, develops max energy in hard driving
4
Rotating eccentric loads cause vertical vibrations, most effective in sand

PILE TYPE
• Timber
• Concrete
• Steel
Timber piles
Butt dia 12" to 20", tip 5" to 10". Length 30-60'
Bark always removed.
Concrete piles
Pre-cast
• Reinforce and prestressed
Cast in place
• With or without casing1
• More common than precast.
Steel Piles
• Steel H-Pile
• Unspliced 140', spliced > 230', load 40 to 120 tons.
1
more economical but more risk in their installation .

PILES CLASSIFICATION
1. Material (wood, steel, etc.).
2. Method of installation1
3. Effect on surrounding soils during installation2
.
Pile load capacity prediction
• Full-scale load test
• Static formulae
• Dynamic method
1
(driven: blow of a hammer, vibrations, pressure from a jack, etc.; jetted, augured, screwing, etc.).
2
displacement, non-displacement

Pile shape effect and pile selection
Shape
characteristics
Pile type placement effect
Displacement Closed ended steel Increase lateral ground stress
Precast concrete Densifies granular soils,
weakens clays1
Tapered Timber,
monotube, thin-
walled shell
High capacity for short
length in granular soils
Non-
displacement
Steel H Minimal disturbance to soil
Open ended steel
pipe
Not suitable for friction piles
in granular soils, often show
low driving resistance, field
varification difficult
resulting in excessive pile
length
1
Seup time for large groups upto 6-months

Load Transfer
s
D
Pile
pQ
L Q
uQ
soil plug
Ap = total plan area
steel
steel
q'
soil plug
Q Q Qu p s= +
Settlement for full load transfer
• Qp →0.1D (driven), 0.25 D (bored)
• Qs →0.2" - 0.3"

End or Point Resistance
qult = cN*c + q'oN*q + γDN*γ
D = pile diameter or width
q'o = effective overburden stress at pile tip
N*c, N*q,N*γ are the b.c. factors which include shape and depth
factors
Since pile dia is small γDN*γ ≈ 0
)NqcN(AQ *
q
'
o
*
cpp +=
Ap = pile tip area

Meyerhof's Method
Point resistance increases with depth reaching a maximum value at
Lb/D critical.
(Lb/D)crit varies with φ and c. Fig 9.12
Lb = embedment length in bearing soil
D
L
Qp
L/D
(L/D)cri
For values of N*c, N*q see Fig 9.13

Piles in Sand1
Q A q N Ap p o
'
q
*
p= ⋅ ⋅ ≤ q⋅ l
where q φ⋅= tan0.5N(tsf) *
ql
φ in terms of 'N' or Dr
o
15N20 +=φ
r
o
D1528 +=φ
Point resistance from SPT
N4
D
LN0.4
=(tsf)qp ⋅≤
⋅⋅
N = avg value for 10D above and 4D below pile tip
10D
4D
Q
1
For a given initial φ unit point resistance for bored piles =1/3 to 1/2 of driven piles, and bulbous piles driven with great impact
energy have upto about twice the unit resistance of driven piles of constant section

Example 1
A pile with L = 65', x-section = 18"×18" is embedded in sand with
φ = 30°, γ = 118.3 pcf. Estimate point bearing resistance.
Solution (Meyerhof)
Q A q N Ap p o
'
q
*
p= ⋅ ⋅ ≤ q⋅ l
tons47620005565118.35.15.1Qp =÷××××=
tons36tan30550.51.51.5qAQ lpp =××××=⋅=

Upper weak lower firm soil
Lb
L
dense
sand
q l(d)
10D
q
l(l)
loose
sand
( ) l(d)l(l)l(d)
b
l(l)p qqq
10D
L
qq ≤−+=
ql(l) = limiting point resistance in loose sand
ql(d) = limiting point resistance in dense sand
Piles in Saturated Clay
If embedded length ≥ 5D, Nc
*
= 9
pup Ac9Q ⋅⋅=

Example 2
Timber piles, 25' log with 10" point diameter, were driven through
a silty sand with φ = 25° into undrelying dense sandy gravel with φ
= 40°. Penetration into the sandy gravel was 3'. Determine point
bearing capacity of a pile.
Solution
ql (silty sand) = 0.5Nq
*
tan φ = 0.5×25 tan 25 = 5.82 tsf
ql (sandy gravel) = 0.5×350× tan 40 = 146.8 tsf
( ) 8.461tsf6.5682.58.461
1010
36
82.5qp <=−
×
+=
tons30.956.6
212
10
Q
2
p =×π×





×
=

Shaft Resistance
It is due to skin friction and adhesion
sAfQs ⋅Σ=
As = area of shaft surface
f = unit shaft resistance
z
L
L'
K
f
D
σ
v
'
f
aoK ctanf '
+= δσ
δ = soil-pile friction angle
ca = is adhesion
K = earth pressure coefficient1
1
which is difficult to evaluate between at-rest and passive state

Shaft Resistance in Sand
Since ca= 0
δσ tanKf '
⋅⋅= o
Like tip resistance a critical depth is reached after which 'f' does not
increase. Use (L'/D)critical =15
Values of K
Pile type K
Bored or jetted Ko = 1 - sin φ
low disp. driven Ko to 1.4 Ko
high disp. driven Ko to 1.8 Ko or
0.5 +0.008 Dr
1
1
Dr = relative density (%)

Pile material δ
steel 20°
concrete 0.75 φ
wood 0.67φ
LfpQs ∆⋅⋅Σ=
p = perimeter
∆L = incremental pile length for constant p and f.

SPT - basis for shaft resistance in sand
Meyerhof
50
N
=(tsf)favg high disp. driven piles
100
N
=(tsf)favg small disp. driven (H-pile)
50
N
1.5=(tsf)favg ⋅ pile tapered > 1%
N avg 'N' within embedded pile length=

Skin Resistance in Clay1
λ Method2
( )u
'
vav 2cf +σλ=
σ v
'
= mean effective vertical stress on pile length
λ- given in Fig. 8.17
LfpQ avs ⋅⋅=
For layered soils use mean values of cu and σ.
C2
C3
L3
L1
L L2 Cu
C1C1
'σ
A3
C v
A2
'σv
A1
L
...+Lc+Lc
=c 2u(2)1u(1)
u
1
some problems 1. increase in pore water pressure 2. Low initial capacity 3. enlarged hole near ground surface-water may get in and
soften clay. 4.ground and pile heaving. 5. Drag down effect from soft upper soils
2
Short pile were driven in stiff clay OCR>1 but the long piles penetrated lower soft clay as well

L
....AAA 321'
v
+++
=σ
A1, A2, ... are areas of the effective stress diagrams

Example 3
A 12 m prestressed concrete pile 450 mm square is installed in a
clay with water table at 5 m depth. Upper clay layer is 5 m thick,
with γ = 17.4 kN/m3
and cu = 50 kPa. Lower clay has γ = 18.1
kN/m3
, cu = 75 kPa. Determine pile capacity using λ - method.
Solution: (λ - method)
'cu
5m
7m
87 kPa75 kPa
145 kPa
vσ
50 kPa
σ5
'
= 17.4×5 = 87 kPa, σ12
'
= 87 + (18.1-9.81)×7 = 145 kPa
64.58kPa
12
775550
cu =
×+×
=
85.79kPa
12
2
14587
75870.5
'
v =
+
×+××
=σ

σcu
64.58 kPa 85.79 kPa
'v-avg
1
λ = 0.24
favg = 0.24(85.79+2×64.58) = 51.6 kPa
Qs = 4×0.45×51.6×12 = 1114 kN
1
Fig 9.17

Example 4. Redo Example 3 using α - method
ucFf ⋅⋅= α
Upper clay, 57.0
87
50c
'
u
==
oσ
cu/σ
α
0.570.35 0.8
0.5
1.0
756.0)57.08.0(
35.08.0
5.01
5.0 =−
−
−
+=α
L/D = 12/0.45 = 26.7, F = 1
Lower clay
517.0
145
75c
'
u
==
oσ
α = 0.814,
f1 = 1×0.756×50 = 37.8 kPa
f2 = 1×0.814×75 = 61.05 kPa
Qs = 4×0.45×5×37.8 +4×0.45×7×61.05 = 1109 kN

Example 5. Redo the Example 3 assuming 200 mm pipe
L/D = 12/0.2 = 60
L/D
1.0
F
6050 120
0.7
( )( )
0.95
50120
601200.71
0.7F =
−
−−
+=
f1 = 0.95×0.756×50 = 35.9 kPa
f2 = 0.95×0.814×75 = 58.0 kPa
Qs = π(0.2) × (5×35.9 +7×58.0) = 368 kN

Example 6
Draw number of blows per inch versus Ru for the following
conditions using EN formula, modified Engineering News formula,
and Janbu formula. Steel HP10×57, coefficient of restitution (n) =
0.8, efficiency (E) = 0.85, Vulcan 08 hammer. C = 0.1". 1
Use two
pile lengths 20' and 80'. Elastic modulus = 29×103
ksi
Solution
Hammer energy 26 k-ft, Ram weight 8 kips. Area of steel = 16.8
in2
.
C 0.75 0.15
57 20
8000
d = +
×
= 0 771.
Assume S = 0.1"
( )
06.13
1.010298.16
26122085.
23
=
×××
×××
=λ
382.40
0.771
06.13
11771.0Ku =





++=
Ru (20 ft for S=0.1") = 657 kips
Ru (80 ft for S=0.1") = 354 kips
1
For plotting graphs assume several Ru and compute set

Bearing Graph
Ru - Janbu 20', Ru1 - Janbu 80', Rmn = Modified EN 20', Rmn1 =
Modified EN 80'
From Modified Engineering New Formula, the following results
will be obtained.
Ru (20 ft for S=0.1") = 1266 kips
Ru (80 ft for S=0.1") = 1153 kips

Practical applications
Design
• Using laboratory and field soil data determine Qu based on static
formulae
Test pile
• Drive test piles using same equipment as proposed for
production piles.
• If possible use pile driving analyser (PDA) incorporating strain
and acceleration data on test pile. Prepare bearing graph.
• Make records of pile driving and correlate with boring logs to
ensure that piles have penetrated the bearing soils.
• Load test the pile/s.
• If possible load to failure to establish actual factor safety.
• For small jobs load tests are not justifiable.
• When piles rest on sound bedrock load tests may not be
necessary.
• Based on load test adjust design capacity, increase penetration
depth, alter driving criteria as necessary.
Construction stage
• Record driving resistance for full depth of penetration
• Driving record must correspond to bearing graph of test pile.

• If not additional penetration into bearing material or greater
driving resistance may be necessary.

Wave Equation
• Hammer, cushions, pile cap, and pile are modeled as discrete
elements.
• Each element has a mass and there are springs between element
of appropriate stiffness
• Soil-pile interface modeled as spring-dash pot. Springs model
resistance to driving as a function of displacement and dash pots
as function of velocity.
Computations
• Ram of mass M1 with velocity v1 travels a distance v1×∆t
compresses spring K1 with the same amount.
• Force in K1 actuates M2 from which displacements are
computed.
• Process is continued for all masses for successive time intervals
until pile tip stops moving.
• Software - TTI and WEAP (Wave Equation Analysis of Pile)

Pile Driving Analyser
• Two strain transducers and two accelerometers mounted near
pile head
• A pile driving analyser (PDA)
PDA monitors strain and acceleration and yields:
• Force in pile - from strain, E and pile x-sectopm
• Particle velocity - from integration of acceleration
• Pile set - from integration of velocity
CAPWAP(Case Pile Wave Analysis Program):
Hammer and accessories - replaced by force-time and velocity-time
time data from PDA, thus eliminating deficiencies of wave
equation.

Pile exmplsolu

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